From the contemporary standpoints of environmental protection and space saving, it is strongly desired to reduce the quantity of spent solution. Needed in this regard is a technology relating to thermographic photosensitive materials for use in medical diagnosis and general photography which can be effectively exposed by means of laser image setters and laser imagers and produce distinct black images having high resolution and sharpness. These thermographic photosensitive materials offer to the customer a simple thermographic system which eliminates a need for solution type chemical agents and is not detrimental to the environment.
On the other hand, the recent rapid progress of semiconductor laser technology has made it possible to reduce the size of medical image output devices. As a matter of course, there were developed techniques relating to infrared-sensitive photothermal silver halide photographic material which can utilize a laser diode as a light source. The spectral sensitization technique is disclosed, for example, in JP-B 10391/1991 and 52387/1994, JP-A 341432/1993, 194781/1994, and 301141/1994. The antihalation technique is disclosed, for example, in JP-A 13295/1995 and U.S. Pat. No. 5,380,635. Since the infrared exposure system permits the visible light absorption of sensitizing dyes and antihalation dyes to be considerably reduced, a substantially colorless photosensitive material can be readily produced.
A combination of the thermographic technology with the infrared exposure technology enables a photosensitive material which eliminates a need for liquid
Since spectral sensitizing dyes capable of absorbing infrared radiation, however, generally have a high reducing power due to a high HOMO (highest occupied molecular orbital), they tend to reduce silver ions in photosensitive materials to exacerbate the fog thereof. In particular, these photosensitive materials experience a substantial change of performance during storage under hot humid conditions and long-term storage. If dyes having a low HOMO are used for preventing the photosensitive material from deteriorating during storage, spectral sensitization efficiency and sensitivity become low because their LUMO (lowest unoccupied molecular orbital) is relatively low. These problems relating to sensitivity, storage stability, and performance change arise not only with wet photographic photosensitive materials, but more outstandingly with photothermographic materials.
The supersensitization technique has been developed for overcoming such infrared sensitization problems. Known infrared supersensitizers for use in thermographic systems include aminopolycarboxylic acid derivatives as disclosed in JP-A 4241/1990, and heteroaromatic mercapto compounds and heteroaromatic disulfide compounds as disclosed in JP-A 182639/1992 and 341432/1993. The aminopolycarboxylic acid derivatives provide weak supersensitization effect and low sensitivity whereas the heteroaromatic mercapto and disulfide compounds allow photographic properties such as sensitivity and gradation to vary with changes of development temperature and time.